The present invention relates generally to the transfer of audio signals between electronic devices. More particularly, the invention relates to a line receiver circuit that reduces the amount of noise introduced into an audio signal as the signal is transferred between physically separated electronic devices.
An audio system typically includes a combination of electronic devices such as preamplifiers, mixers, power amplifiers, etc. In an audio system designed for a large listening audience such as in a convention hall, theater, or building, such electronic devices might be physically separated by large distances and powered by different ac power line circuits. The different ac power line circuits can introduce significant undesired noise to the audio signal.
Each ac power line circuit has its own local ground reference. The capacitance between the ac power line and the chassis of each electronic device can induce a voltage at the local ground of the chassis. Thus, a considerable voltage difference can arise between the local grounds of the various electronic devices of the audio system. This voltage difference causes a current to flow in any conductor that interconnects electronic devices connected to different ac power line circuits. These currents that flow through the conductor between the electronic devices are often rich in harmonics of the power line frequency, which is typically 50 to 60 hertz. These harmonics of the power line frequency are heard as a distracting hum or buzz, if reproduced by the audio system.
With reference to FIG. 1, a prior art audio system 10 is shown to include a driving device 12 and a receiving device 14. An audio signal Vs is transmitted from the driving device to the receiving device over a cable 16 that includes a conductor pair having first and second differential lines 18 and 20, respectively. The cable may also include a shield 21 that surrounds the first and second differential lines. Typically the shield is connected to the local ground 24 of the driving device. The audio signal Vs is transmitted in a differential mode, which means that the audio signal is represented by the difference in voltage between the differential lines 18 and 20 irrespective of any local ground reference. The driving device has a source impedance associated with its positive and negative output designated Rs+ and Rs-, respectively. The receiving device includes a line receiver 22 which has an input impedance associated with its positive and negative input terminals, designated IN+ and IN-, respectively.
The noise voltage Vn caused by the voltage difference between the local grounds 24 and 26 of the respective driving device 12 and receiving device 14 is presented simultaneously to both input terminals IN+ and IN- of the line receiver 22. This noise voltage Vn is commonly referred to as a common-mode voltage. A common-mode voltage is a voltage referenced to the local ground of the receiving device that is present at both input terminals IN+ and IN- of the line receiver 22. The common-mode noise voltage Vn contains the undesired harmonics of the power line frequency and it is very desirable for the receiving device to reject such common-mode voltage signals.
As shown in FIG. 2, one method for the line receiver 22' to reject the common-mode voltage signals is to use a transformer T1 coupled to the inputs of the receiving device 14. The common-mode noise voltage, which is applied simultaneously to both inputs IN+ and IN- of the primary winding 28, is not transmitted from the primary winding to the secondary winding 30 of the transformer, but the differential-mode audio signal is transmitted. Accordingly, the amplifier 31 receives only the differential-made audio signal. Although the transformer therefore is effective in eliminating common-mode noise voltage, Vn, it has cost, size, and weight disadvantages that limit its widespread use.
With reference to FIG. 3, a simple voltage-mode differential amplifier circuit 32 has been used in the past as a line receiver that eliminates common-mode noise without using a transformer. The circuit 32 includes an operational amplifier A1 and four gain/bias resistors R1, R2, R3 and R4. The circuit 32 has two input terminals IN+ and IN- and an output terminal OUT. The voltage gain for voltage signals applied to the input terminal IN- is -R4/R2. The voltage gain for voltage signals applied to the input terminal IN+ is [(R2+R4)/R2] [R3/(R1+R3)]. By setting R1 equal to R2 and R3 equal to R4, the voltage gain for voltage signals applied to the input terminal IN+ becomes R4/R2. The voltage gain for common-mode voltage signals is found by summing the voltage gain of the two inputs together. Thus, the gain of the differential amplifier circuit 32 for common-mode voltage signals is zero, if resistors R1 and R3 are matched to resistors R2 and R4 respectively, since (R4/R2)+(-R4/R2)=0. Thus, by carefully matching the resistor values, any common-mode voltages at the input terminals IN+ and IN- are substantially rejected.
In actual practice, the resistors R1 and R2 and the resistors R3 and R4 are never perfectly matched, and the rejection of common-mode voltage signals is quantified as the common-mode rejection ratio (CMRR). The CMRR of a differential amplifier circuit is defined as the gain of differential-mode signals divided by the gain of common-mode signals. In addition, the effects of the output impedances Rs+ and Rs- of the driving device 12 which have not yet been discussed, can adversely affect CMRR.
With reference now to FIG. 4, a simple current-mode differential amplifier circuit 34 also has been used in the past as a line receiver that eliminates common-mode noise without using a transformer. The circuit 34 includes two operational amplifiers A2 and A3, and five gain/bias resistors R5, R6, R7, R8 and R9. Like the voltage-mode circuit discussed above, this circuit has two input terminals IN+ and IN- and an output terminal OUT. The inverting terminal of the operational amplifier A3 acts as a current summing node at virtual ground. The current through the resistors R7 and R8 is determined by the classic ohm's law formula I=V/R. The current through the resistor R9 is merely the negative sum of the currents through the resistors R7 and R8. To have the amplifier operate in a differential mode, one input, in this case the input associated with the resistor R7, is inverted by the inverting amplifier configuration of the resistors R5 and R6 and the operational amplifier A2.
The resistor pair R5 and R6 and the resistor pair R7 and R8 should be matched to obtain a large CMRR. In actual practice, the resistor pair R5 and R6 and the resistor pair R7 and R8 are never perfectly matched, which results in a finite CMRR as discussed above with respect to the voltage-mode differential amplifier.
The differential amplifier circuits 32 and 34 shown in FIGS. 3 and 4 offer good common-mode voltage rejection only if the source impedances Rs+ and Rs- of the driving device 12 are very closely matched. If the source impedances are not closely matched, the circuit's CMRR degrades significantly. The CMRR degrades because these circuits have relatively low common-mode input impedances at their two input terminals IN+ and IN-. These low input impedances are in series with the source impedances. If the source impedances are not exactly equal, an unequal voltage division of the audio signal occurs at the input terminals IN+ and IN-. For example, in practical application of the circuits 32 and 34 shown in FIGS. 3 and 4, a source impedance imbalance of only 5 ohms will degrade the CMRR from 80 db to 40 db.
With reference now to FIG. 5, a more complex instrumentation amplifier circuit 36 can substantially eliminate the sensitivity of the simple differential amplifier circuits 32 and 34 to unbalanced source impedances of the driving device 12. The instrumentation amplifier circuit 36 has two input buffers 38 and 40 connected to the respective inputs of the differential amplifier, shown here as a voltage-mode differential amplifier 32. Each input buffer is typically an operational amplifier A4 configured for unity gain. The large input impedance of the operational amplifier reduces to negligible levels the voltage division effects caused by the source impedance. The output impedance of the operational amplifier is very small and does not vary significantly over a large range of output current levels. Thus, the input buffers 38 and 40 prevent the unbalanced source impedances of a driving device from reducing the CMRR of the differential amplifiers 32 and 34.
An instrumentation amplifier 42 shown in FIG. 6, is known to eliminate common-mode noise. The amplifier includes two operational amplifiers A5 and A6 and five gain/bias resistors R10, R11, R12, R13 and R14. By matching the gain/bias resistors, the circuit exhibits unity gain for differential-mode signals and zero gain for common-mode signals. However, as mentioned above, the gain/bias resistors are never perfectly matched and the amplifier exhibits a finite CMRR.
An improved instrumentation amplifier 44, shown in FIG. 7, also is known to improve the performance of a simple differential amplifier 46, such as the differential amplifiers 32 and 34 shown in FIGS. 3 and 4, respectively. The instrumentation amplifier has a differential-gain stage 45 between the input terminals IN+ and IN- of the instrumentation amplifier and the input terminals IN+' and IN-' of the differential amplifier 46. The differential-gain stage includes two identical circuits 48, each circuit including an operational amplifier A7 and a bias/gain resistor R15, and each connected to a separate input terminal. The noninverting input of one operational amplifier A7 is connected to the input terminal IN+, and the noninverting input of the other operational amplifier A7 is connected to the input terminal IN-. A resistor R15 is connected between the noninverting input and the output of each amplifier A7, and a resistor R16 is connected between the inverting inputs of the two operational amplifiers. The outputs of the two operational amplifiers A7 are connected to the respective inputs IN+' and IN-' of the differential amplifier 46. The differential amplifier 46 may take the form of either the voltage-mode differential amplifier 32 shown in FIG. 3 or the current-mode differential amplifier 34 shown in FIG. 4.
The differential-gain stage amplifies differential mode signals by a factor equal to (2R15+R16)/R16 and amplifies common-mode signals by unity. Thus, the differential amplifier 46 receives a signal in which the common-mode signal has already been suppressed by a factor of (2R15+R16)/R16, thereby improving the CMRR of the instrumentation amplifier 44 over that of the simple differential amplifier.
However, each of the instrumentation amplifier circuits 36, 42 and 44 shown in FIGS. 5, 6 and 7, respectively, has a serious practical problem when used as a line receiver in an audio system, because the inputs of the operational amplifiers may have no external dc paths for their bias currents. Such dc bias currents are necessary for the operational amplifiers to function properly. Since the signal sources of an audio system are often ac coupled, they cannot be relied upon to provide the dc path. In addition, using a resistor connected to a local ground terminal to provide the dc path will degrade the CMRR of these instrument amplifier circuits by lowering the input impedance of each input buffer.
It should therefore be appreciated that there is the need for amplifier circuits that, when used as a differential audio line receiver, provide a very large common-mode input impedance while at the same time providing a dc path for the bias currents of the operational amplifiers. The present invention fulfills this need.